For example, document 1 (JP 2011-108669 A) discloses a prior lighting apparatus for an LED exemplifying a semiconductor light emitting element.
As shown in FIG. 8, this prior instance is an LED dimming apparatus which includes: a current adjustment means (a constant current circuit 6) configured to vary a magnitude of a current flowing through an LED load 4; a switching means (a transistor Q1) configured to turn on and off the current flowing through the LED load 4; and a dimming control means (a microcomputer 5) configured to control the current adjustment means and the switching means in response to a dimming signal outputted from a dimmer 1. This LED dimming apparatus is characterized by the dimming control means. The dimming control means is designed to, when the dimming signal outputted from the dimmer 1 indicates luminance higher than that corresponding to a predetermined level, supply a continuous current to the LED load 4 and to dim the LED load 4 by means of adjusting the magnitude of the continuous current. The dimming control means is designed to, when the dimming signal outputted from the dimmer 1 indicates luminance lower than that corresponding to the predetermined level, supply a pulse current to the LED load 4 and to dim the LED load 4 by means of adjusting a duty ratio of a waveform of the pulse current.
When the LED load is not so dimmed but is relatively bright, the lighting apparatus changes the magnitude of the current flowing through the LED load. When the LED load is dimmed and is relatively dark, the lighting apparatus changes the duty ratio of the waveform of the pulse current. Thereby, the light apparatus can suppress noises while the LED load is relatively bright and can successfully maintain the luminance of the LED load even when the LED load is relatively dark.
With regard to a process of dimming the LED load by means of adjusting the magnitude of the current flowing through the LED load (hereinafter referred to as “DC dimming process”), it is difficult to dim the LED load at the lowered dimming level because individual V-I characteristics of LED elements are different and it is necessary to keep the current supplied to the LED elements not less than a predetermined current selected such that all the LED elements light successfully.
With regard to a process of dimming the LED load by means of adjusting the duty ratio of the waveform of the pulse current flowing through the LED load (hereinafter referred to as “PWM dimming process”), it is necessary to narrow a pulse width in order to dim the LED load at the lowered dimming level. It is difficult to stably control the LED load because a small change in the pulse width causes flickering of the LED load.
Consequently, it is not easy to dim the LED load at the lowered dimming level (e.g., the dimming ratio is equal to or less than 1%).